In the continuous casting of steel products, which may be in the form of a billet, bloom or slab, molten steel is delivered to the upper end of a vertical casting mold of the dimensions desired for the product. As the steel descends in the mold, it commences to solidify from the exterior towards the interior. While still in a pliable state, the solidifying steel is guided through a curved path to a horizontal direction.
The operating characteristics of the continuous casting procedure need to be known and under close control to maintain safe, efficient continuous casting. Process control is verified by evaluating the internal quality in at least the cross-section and at other times the longitudinal section of the cast steel. Steel is considered to have satisfactory internal structure if there are no internal cracks, no internal voids, no internal porosity, no inclusions and internal symmetry of zones of solidification.
Immediately after the product is solid, a sample can be cut from the cross-section and, after surface preparation, the sample is tested by either or each of two conventional methods, namely sulphur printing or acid etching. If the sulfur content of the steel is less than 0.010% S or deoxidized with aluminum, only the acid etching method is workable.
Conventional acid etching procedures are time consuming and unreliable in providing a rapid processing of a steel sample to reveal its internal quality. Such acid etching (ASTM Standard E381-79) generally involves selective attack on the metal surface by an aqueous acid solution comprising 1 to 1 v/v technical grade hydrochloric acid at about 70.degree. to 80.degree. C. for longer than about 20 minutes, the time depending on the initial temperature of the metal, followed by visual inspection of the etched surface.
Electrochemical etching and electropolishing of small metal specimens is part of the existing art of chemical analysis and metallography. For example, U.S. Pat. No. 4,533,642, assigned to the assignee hereof, describes an electrolytic etching procedure for determining the acid-soluble aluminum content of small steel samples. This procedure employs small quantities of steel to determine the specific content of aluminum by chemical analysis of the spent etchant. The electrolytic etching of large scale metal samples does not appear to have been practiced previously and not for the purpose of determining the internal quality of a steel sample, as is effected herein.
One of us, John H. Kelly, is aware, by private communication, the Inco Ltd, in the UK, has utilized an electrolytic etching process on nickel and nickel alloys to determine the internal quality of samples of the metal. Details of the process are trade secret but such information as is known to me indicates a process using stronger acid and a lower current density than is employed herein. These parameters suggest to me that the application of current is secondary tot he process and etching is effected mainly by chemical reaction with the acid. In contrast, the etching procedure described herein is truly coulometric, requiring current flow to effect etching, since the etchant itself is unable significantly to react with the sample surface in the absence of such current.